The Boeing 737-800 can be flown either manually or automatically. This also applies to themanagement of the engines. The autothrottle regulates the thrust of the engines. The aircraft isfitted with two radio altimeter systems, one on the left and one on the right. In principle, the autothrottleuses the altitude measurements provided by the left radio altimeter system. Only if thereis an error in the left system that is recognised as such by the system, the autothrottle will use theright-hand radio altimeter system.

In a worst-case scenario, the left-side altimeter tells the autothrottle that the aircraft is lower than it is while the right-side autopilot is flying the plane, but the left altimeter thinks its data is good. Fly low enough in this condition, such as a final approach, and data from the left altimeter can roll the throttles back to idle while the right-side autopilot, relying on accurate data from its altimeter, will keep pushing the nose up to maintain the glideslope.

If this happens at too low an altitude and the pilots aren't quick enough to recover from an unexpected aircraft state, the results can be devastating.

This is what happened to Turkish Flight 1951. Not for nothing, the flight's crew included a captain acting as a flight instructor alongside the less experienced first officer, as well as a "safety pilot" who, among other things, was supposed to help the captain--tasked with a few extra instructional duties--keep an eye on the aircraft state.

The three pilots, one crewmember, and five passengers died in the accident.

Photo: Dutch Safety Board

Again, from the report:

This [design] arelic from the Boeing 737, certificated long ago, which in the original design prioritised the provisionof information to the left pilot (captain). This original design has now been superseded by bothtechnical facilities and a democratisation and reallocation of pilot duties in the cockpit. It is noticeablethat this subject cannot be found in any of the Boeing 737 manuals or training documents forpilots. Pilots therefore do not have the correct knowledge about links between the control systemsand data input for their own aircraft. The result of this is an incomplete or even incorrect ‘mentalmodel’ of the automated flight control.

FAA's AD is based on Boeing service bulletin issued in November. The directive will require autothrottle computer replacements or modifications on affected 737NGs, which FAA says number 497 in the U.S. alone.

"The service bulletin referenced in the NPRM is one of the final components of the changes Boeing has made," a Boeing spokesman notes.

Others include operations manual updates and software changes in April 2010, and a July 2010 production line cut-in that added an AIRSPEED LOW aural alert to new 737NGs.